Spark plug with insulator nose spaced from center electrode



June 14, 1966 M. A. BRETSCH SPARK PLUG WITH INSULATOR NOSE SPACED FROMCENTER ELECTRODE Filed July 13, 1961 INVENTOR. MICHAEL A. BRETSCHATTORNEYS United States Patent 3,256,457 SPARK PLUG WiTH FLYSULATOR NOSESPACED FROM CENTER ELECTRGDE Michael A. Bretsch, Toledo, Ohio, assignorto Champion Spark Plug Company, Toledo, Ohio, a corporation of DelawareFiled July 13, 1961, Ser. N 123,878 Claims. (Cl. 313-141) This inventionrelates broadly to spark plugs, and, more particularly, to ceramicinsulated spark plug assemblies highly resistant to insulator crackingcaused by thermal gradients brought about during normal service usethereof, and to methods of producing such assemblies.

In the use of ceramic insulated spark plugs for high output internalcombustion engines, including aircraft engines, substantial diflicultyhas been encountered because the insulators have had a tendency to crackat the nose portion. The nose portion is that part of the spark pluginsulator surrounding the center electrode that extends inwardly towardsthe firing end from the seat on'which the insulator rests when the sparkplug is assembled. When the nose portion cracks the resultingdiscontinuity often causes a drastic temperature rise, with the resultthat the tip of the spark plug may run so hot as to cause pro-ignitionin the associated cylinder and a possible failure of the engine.Pre-ignition is a serious difiiculty in a high output internalcombustion engine. If such an engine continues operation in pre-ignitionfor more than a few seconds, the extreme heat generated will damage thespark plug, and the engine. If pre-ignition does not occur afterformation of a crack in the nose portion of a spark plug insulator, thecrack may become a complete fracture and a part of the insulator may bedislodged. The dislodged part may itself cause severe mechanical damagein the combustion chamber.

In addition to spark plug failure caused by pre-ignition resulting fromnose cracking as described above, pre-ignition with resulting spark plugfailure often results when the engine output surpasses that theparticular spark plug can withstand. When a spark plug is operated in ahigh output engine, and the output of that engine is graduallyincreased, an engine output is reached at which that particular sparkplug causes pre-ignition.

The indicated means effective pressure of an engine at which a sparkplug, when operating within the engine, will cause pre-ignition, iscommonly denominated the I.M.E.P. rating of that spark plug. It has beenfound that the I.M.E.P. rating of a spark plug can be increased byshortening the nose thereof, but that such expedient is relativelyundesirable because such plugs are particularly subject to carbonfouling when operated at low output. It has also been found that theI.M.E.P. rating of a spark plug can be effectively increased without.

deleteriously affecting the susceptibility of the plug to foulding bycasting a silver electrode part in a bore through the nose portion ofthe spark plug insulator. Such expedient, however, increases thesusceptibility of the insulator to nose cracking and, therefore, is notdesirable. As a consequence, an acute problem exists in previously knownspark plug designs: that of finding a spark plug having a high I.M.E.P.rating which is not subject to substantial carbon fouling, while at thesame time is capable of operation in high output engines without atendency to crack at the nose portion.

It has now been discovered, and the instant invention is based upon suchdiscovery, that the advantages of the cast in place, silver. electrodestructure, can be achieved, and the tendency for the cast silver tocause nose cracking substantially eliminated or at least mini- PatentedJune 14, 1966 ice mized by casting a metal having a high thermalconductivity, for example silver, into a metallic sleeve or tube havinga relatively low thermal conductivity in comparison with the cast metal,such sleeve or tube being disposed in at least a part of the borethrough the nose of the insulator. In a specific. instance, excellentresults have been achieved where a split nickel sleeve, before castingof thesilver, has been expanded against the wall of the insulator bore;in such case, after casting of the silver or other high thermalconductivity metal, and because of the contraction of such metal duringcooling thereof from a molten condition, an air gap is establishedbetween the Wall of the insulator bore and the metallic sleeve, with theresult that the sleeve fits loosely within the insulator bore, and, infact, may even slide therein when the assembly is tipped. It has beenfound that, in service, the nickel tube or sleeve, constituting amaterial having a low thermal conductivity relative to that of the castsilver body, and the gas filled gap be tween the exterior of the tubeand the insulator nose bore wall, constitute thermal barriers to thetransfer of heat between the nose of the insulator and the electrodepart, whereby as a consequence of the thermal barriers, stressing of thenose bore wall by the cast silver body to an extent sufiicient to causeinsulator cracking is prevented, but, at the same time, heat transferbetween the two is sutiiciently high that the spark plug has a highI.M.E.P. rating.

It is, therefore, an object of the invention to provide an improvedspark plug assembly.

It is a further object of the invention to provide such an assemblywhich will operate satisfactorily in engines running under a high load,or at a high I.M.E.P. Without insulator nose cracking.

More particularly, it is a further object of the invention to provide aspark plug assembly which includes a body of a material having a highthermal conductivity, at least part of which body is within a bore ofthe insulator, and means disposed within an annular space between thebody and the wall of the insulator defining the nose bore, which meansis effective to decrease the rate of heat transfer between the body andthe nose bore wall.

It is a still further object of the invention to provide a method forproducing such a spark plug assembly.

Other objects and advantages will in part be apparent and will in partappear hereinafter.

For a better understanding of the nature and objects of the invention,reference should be had to the following detailed description andattached drawing, illustrating a preferred embodiment thereof, in which:

FIG. 1 is a vertical sectional view of a spark plug assembly embodyingthe present invention; and

FIG. 2 is a horizontal sectional view taken along the line 22 of FIG. 1.

According to the invention, an improved spark plug assembly is provided.Such assembly, as shown in the drawings, comprises an appropriateceramic insulator indicated generally at 11. The insulator is engaged inand supported by a metal shell indicated generally at 12 which isthreaded at 13 for engagement in the combustion chamber of an engine.The insulator 11 has a nose portion 14 extending from its firing end toan intermediate area indicated generally at 15 which is operativelyassociated with the metal shell 12 for conductive heat transfer betweenthe two. The insulator 11 has a central bore indicated generally at 16and formed in steps as indicated at 17 and 18. The step 18 is providedin the bore through an intermediate insulator portion 19 so that thebore diameter defined by a nose bore wall 20 is less adjacent the firingend of the insulator than through the intermediate portion. A splitmetallic tube or sleeve 21, which in the specific embodiment is nickel,is disposed in the smaller diameter insulator bore portion, adjacent theinsulator firing end, between the nose bore wall 20 and an electrodepart indicated generally at 22. The electrode part 22 includes a firingtip 23, the lower extremity of which constitutes a sparking surfaceoperatively associated with a ground electrode 24 which is structurallyintegral with the metal shell 12. The electrode part 22 also includes abody 25 of cast metal, which in the particular embodiment is silver. Thebody 25 is in both mechanical and electrical contact with the firing tip23, and also with an upper electrode part 26. The upper electrode part26 extends upwardly through a suitable gastight seal 27 and makeselectrical contact with a resistor part 28 which is urged thereagainstby a spring 29 that is confined within a cap member 30. The nickelsleeve 21 is positioned in closely adjacent, spaced relationshiprelative to the wall of the insulator defining the nose bore. Foroptimum plug performance the spacing is kept to a minimum, andpreferably is not greater than about 0.001". Excellent results have beenachieved when this spacing has been approximately 0.0005". This spaceconstitutes a gas-filled gap between the exterior of the sleeve and theinsulator nose-bore wall 20.

The assembly illustrated in'the drawing can be readily produced byinserting the split nickel tube or sleeve 21 into the bore of theinsulator in the position shown in FIG. 1. The sleeve 21 preferably hasan exterior diameter slightly greater than the interior diameter of thebore in which it is inserted so that it is compressed slightly when inposition. After insertion, the sleeve is turned outward at its upper endto conform to the stepped insulator bore at this point, and to form amechanical lock preventing the downward movement of the sleeve. Thefiring tip 23, preferably made of platinum or other corrosion resistantmetal, is then dropped into the sleeve with one end protruding throughthe hole in the sleeve bottom. A rod of silver is next positioned abovethe firing tip with a washer 31 and the upper electrode part 26positioned thereabove. The resulting assembly is then placed in asuitable furnace and heated to a temperature sufficiently high to fusethe silver rod (melting point 960 C.), and thereafter cooledsufficiently slowly to prevent cracking. The silver, upon fusion, ineffect forms a brazing-type of bond with the nickel sleeve so as tobecome integral therewith, no substantial alloying, however, takingplace between the metals. Upon cooling, the silver, of course,contracts, and pulls the split nickel sleeve into a smaller diameterwhereby the air filled gap is formed between the outer diameter of thesleeve and the insulator nose bore wall. The gas-tight seal 27 is thenformed in any suitable way; the insulator is inserted in the metal shell12, with suitable gasketing, and the shell is crimped into engaging andsupporting relationship relative to the insulator; and the remainder ofthe assembly is completed in a conventional manner.

It will be apparent from the above description that a metallic sleeve,having a lower thermal conductivity than the cast-in-place electrodepart, is disposed between the cast metallic body and the wall of theinsulator which defines the bore thru the nose thereof, or the nose borewall. It will be appreciated that the sleeve or tube material, by virtueof the difference in thermal conductivity, constitutes a thermal barrierbetween the cast electrode part and the ceramic insulator. In additionto the sleeve or tube itself serving as a thermal barrier between thecast electrode part and the ceramic insulator, the air gap, formed bycontraction of the silver as described, serves as a further barrier tothe transmission of heat between the cast electrode part and theinsulator. However, neither of the thermal barriers above described isof sufficient magnitude to lower the I.M.E.P. rating appreciably belowthat obtained with a conventional cast silver electrode assembly withoutsuch barriers. In other words, even though the thermal barrier resultingfrom the sleeve and air gap in spark plug assemblies produced inaccordance with the invention is effective substantially to preventinsulator nose cracking, still the sleeve material has a sufliciently hih thermal conductivity that heat transfer between the electrode andinsulator is high, and the spark plug assembly has a high I.M.E.P.rating.

It has been found that substantially equivalent results can be achievedby what may be denominated a reversal of the air gap relative to thesplit sleeve. For example, the interior of the sleeve can first beoxidized to prevent adhesion of silver or the like thereto, and can thenbe positioned in the insulator bore as described, with a suitablebonding material disposed between its exterior wall and the insulatorbore. This bonding material can be silver, or any suitable ceramicbonding material. In this instance, when the assembly is heated, withsilver in posito flow into the split nickel sleeve upon melting, thesleeve is bonded to the insulator bore but the oxide coating on theinterior of the tube prevents adhesion with the silver therein, with theresult that contraction upon cooling provides an air gap to act as athermal barrier between the body of silver and the nickel sleeve. Bycarrying out the procedure which has been described in this paragraphexcept that the silver or other bonding material is omitted from betweenthe exterior of the nickel sleeve and the insulator bore, a stillfurther modified result can be achieved: a limited thermal barrierbetween the exterior of the nickel sleeve and the insulator bore and agreater thermal barrier between the silver body and the interior of thenickel sleeve.

The sleeve or tube is also effective to prevent direct contact betweenthe cast body and the nose bore wall, and, therefore, when the assemblyis heated in service, prevents stressing to an extent which would besufiicient to cause insulator cracking, of the nose bore wall by theexpanded cast body.

As will be readily appreciated, the sleeve material, in addition tohaving a suitable thermal conductivity, must possess a certainrefractory or mold-like property which enables a metal having a highthermal conductivity and forming the electrode part, for example,silver, to be cast therein without causing either a chemical or physicalchange in the structure of the sleeve material. For example, the sleevematerial must have a considerably higher melting point thanthe castmetal, and must be inert'with respect to such cast metal at thetemperatures employed for casting so that substantially no interfacealloying, which would tend to materially lower the thermal conductivityof the cast electrode part, takes place. As previously indicated, nickelhas been found to be especially well suited for use as the sleevematerial. In addition to nickel, other specific materials that may bementioned as particularly well adapted for use as the sleeve materialare certain nickel alloys such as D Nickel percent nickel, 4.5 percentmanganese) and Inconel (76 percent nickel, 13 percent chromium, 7percent iron and other minor constituents, as well as chrominum,molybdenum, and iron). In general, high temperature metals and alloyshaving a thermal conductivity not greater than C.G.S. are preferred.Thermal conductivity as used herein and in the appended claims, isdefined as the quantity of heat in calories which, at room temperatureor approximately 18 C. to 22 C., is transmitted per second through aplate one centimeter thick across an area of one square centimeter whenthe temperature difference is 1 C.

As has hereinbefore been indicated, the cast material must be one thatpossesses an extremely high thermal conductivity. In addition to silver,excellent results may be obtained with aluminum, copper, and gold,employed as the cast electrode part. In this respect, it has been foundthat the cast material must have a thermal conductivity of at least 0.5.Therefore, any metal or alloy capable of being cast under normalconditions, and having a thermal conductivity, as measured in the abovemanner, of at least 0.5 is suitable for use as the cast electrodematerial in the present invention.

Itwill be appreciated that an insulator can stand a certain magnitude ofthermal stress in service without cracking, but that the magnitude ofthermal stress which causes cracking depends upon the composition of theinsulator. For example, certain insulators containing large proportionsof BeO are capable of withstanding much higher thermal stresses, withoutcracking, than are more conventional insulators containing at least 90per cent of A1 0 High alumina insulators, however, are considerably moreresistant to cracking when subjected to thermal stress than are stillother insulators that have been used or might be used. In its essentialdetails, the instant invention contemplates a thermal barrier (thesleeve or tube and air gap) disposed between a body of a material havinga high thermal conductivity (for example, silver) and an insulator nosebore wall. Ideally, the magnitude of the thermal barrier should be justsufiicient to prevent cracking of the particular insulator by thermalstress encountered in service. An assembly, including such a thermalbarrier, has a maximum I.M.E.P. rating, for a given design, for theparticular insulator material. However, a lesser thermal barrier wouldsuffice to prevent nose cracking of an insulator capable of withstandinga high thermal stress, and a greater thermal barrier would be requiredto prevent nose cracking of an insulator which could withstand only asmaller stress. Accordingly, an assembly according to the invention mustinclude a thermal barrier of sufiicient magnitude to prevent nosecracking, but the magnitude thereof cannot be defined numericallybecause it depends upon other factors.

It will be apparent from the foregoing description of the method forproducing a spark plug assembly according to the invention that a newand useful method improvernent as well as a new and useful spark plugassembly is provided. Such method is for producing a spark plug assemblycomprising an insulator that is to be engaged and supported by a metalshell, the insulator having a terminal portion and a nose including afiring end, the nose extending from the firing end to an intermediateinsulator area adapted for operative association with the shell forconductive heat transfer between the shell and the sleeve or tube, thenose having a bore defined by a nose bore wall, and a terminal portionhaving a bore axially aligned with the nose bore.

The method includes the steps of forming at least a portion of anelectrodepart by casting a metal or metal alloy having a thermalconductivity of at least 0.5 within the insulator nose bore, andadditionally supporting in the insulator nose bore, prior to the castingstep, a split metallic sleeve or tube of a material having a relativelylow thermal conductivity in comparison with the cast electrode part,which sleeve or tube is effective substantially to prevent directcontact between the insulator nose bore wall and the cast material, bothduring casting and subsequently, and which is also effective to spacethe cast body from the nose bore wall a'distance sufficient that theinsulator does not crack under service conditions when in the combustionchamber of an internal combustion engine operated from an idle conditionto a high-load condition at which pro-ignition occurs. The spacingprovided should also be sufiiciently small that the cast body iseffective to increase the I.M.E.P. rating of a spark plug assembled fromthe insulator structure.

While it is believed that the more advantageous embodiments of theinvention have been described, it is apparent that many modificationsand variations can be made in the specific construction, arrangement, orform of the parts, and in specific procedure discussed without departingfrom the spirit and scope of the present invention, as those skilled inthe art will readily understand. Such modifications and variations areconsidered to be 6. within the purview and scope of the inventionasdefined by the appended claims.

What I claim:

1. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedand supported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend to an intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore wall, and the terminal portion having a boreaxially aligned with the nose bore, an electrode part disposed in thenose bore and having a sparking surface in the vicinity of the firingend of said insulator, said electrode part including a cast body of amaterial having a thermal conductivity of at least 0.5, at least a partof said body being within the nose bore and in closely adjacent, spacedrelationship to the nose bore wall, and means disposed within an annularspace between said body and the nose bore wall, structurally integralwith said body and out of contact with and spaced from said nose borewall by no more than about 0.001 inch, said means comprising a materialhaving a relatively low thermal conductivity in comparison with that ofsaid body and being effective to decrease the rate of heat transferbetween said body and the nose bore wall in an amount sutficient toprevent stressing of the nose bore wall by said body. when the assemblyis heated in service, to an extent sufiicient to cause insulatorcracking.

2. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamberof an engine, an insulator engagedand sup-ported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend to an intermediate insulator area operativelv associated with saidshell for conductive heat transfer between the two, the nose having abore defined by a nose bore wall, and the terminal portion having a boreaxially aligned with the nose bore, an electrode part disposed in thenose bore and having a sparking surface at the firing end of saidinsulator, said electrode part including a cast body of a materialhaving a thermal conductivity of at least 0.5, at least a part of saidbody being within the nose bore and in closely adjacent, spacedrelationship to the nose bore wall, and a tube disposed within anannular space between said body and the nose bore wall, said tube beingout of contact with. and spaced from the nose bore wall by no more thanabout 0.001 inch and being structurally integral with said body, .andbeing comprised of a material having a relatively low thermalconductivity in comparison with that of said body, whereby, when theassembly is heated in service, stressing of the nose bore wall by saidbody to an extent sufiicient to cause insulator cracking is prevented.

3. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedin and supported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend to an intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore wall, and the terminal portion having a boreaxially aligned with the nose bore, an electrode part disposed in thenose bore and having a sparking surface at the firing end of saidinsulator, said electrode part including a cast silver body, at least apart of said body being in the nose bore and in closely adjacent spacedrelationship to the nose bore wall, and means disposed within an annularspace between said body and the nose bore wall, in substantiallycontinuous contact with one and out of contact with and spaced not morethan about 0.001 inch from the other, said means comprising a materialhaving a lower thermal conductivity than said cast silver body and beingeffective to decrease the rate of heattransfer between said body and thenose bore wall in an amount sufficient to prevent stressing of the nosebore wall by said body, when the assembly is heated in service, to anextent sufficient to cause insulator cracking.

4. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedand supported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend into an intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore .wall, and the terminal portion having abore axially aligned with the nose bore, an electrode part disposed inthe nose bore and having a sparking surface at the firing end of saidinsulator, said electrode part including a cast copper body, at least apart of said body being within the nose bore and in closely adjacent,spaced relationship to the nose bore wall, and means disposed within anannular space between said body and the nose bore wall, structurallyintegral with said body and out of contact with and spaced from saidnose bore wall by approximately 0.001 inch, said means comprising amaterial having a lower thermal conductivity than said east copper bodyand being effective to decrease the rate of heat transfer between saidbody and the nose bore wall in an amount sufiicent to prevent stressingof the nose bore wall by said body, when the assembly is heated inservice, to an extent sufficient to cause insulator cracking.

5. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedand supported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend into a intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore -wall, and the terminal portion having abore axially aligned with the nose bore, an electrode part disposed inthe nose bore and having a sparking surface at the firing end of saidinsulator, said electrode pant including a cast gold body, at least apant of said body being within the nose bore and in closely adjacent,spaced relationship to the nose bore wall, and means disposed within anannular space between said body and .the nose bore wall, structurallyintegral with said body and out of contact with and spaced from saidnose bore wall by approximately 0.001 inch, said means comprising amaterial having a lower thermal conductivity than said cast gold bodyand being effective to decrease the rate of heat transfer between saidbody and the nose bore wall in an amount sufficient to prevent stressingof the nose bore :wall by said body, when the assembly is heated inservice, to an extent sufiicient to causeinsulator cracking.

6. A spark plug assembly compising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedand supported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend into an intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore wall, and the terminal portion having a boreaxially aligned with the nose bore, an electrode part disposed in thenose bore and having a sparking surface at the firing end of saidinsulator, said electrode part including a cast aluminum body, at leasta part of said body being within the nose bore and in closely adjacent,spaced relationship to the nose bore wall, and means disposed w-ithin anannular space between said body and the nose bore wall, structurallyintegral with said body and out of contact with and spaced from saidnose bore wall by approximately 0.001 inch, said means comprising amaterial having a lower thermal conductivity than said cast aluminumbody and being effective to decrease the rate of heat transfer betweensaid body and the nose bore wall in an amount sufficient to preventstressing of the nose bore wall by said body, when the assembly isheated in service, to an extent sufficient to cause insulator cracking.

7. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedand supponted by said she-ll, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend to an intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore wall, and a terminal portion having a boreaxially aligned with the nose bore, an electrode part disposed in thenose bore and having a sparking surface at the firing end of saidinsulator, said electrode part including a cast body of a materialhaving a thermal conductivity of at least 0.5, at least a part of saidbody being within the nose bore and in closely adjacent spacedrelationship to the nose bore wall, and a nickel sleeve disposed withinan annular space between said body and the nose bore wall, out ofcontact with and spaced from the nose bore wall by not more thanapproximately 0.001 inch and structurally integral with said body,whereby when the assembly is heated in service, stressing of the nosebore wall by said body to an extent sufficient to cause insulatorcracking is prevented.

8. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedand supported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend to an intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore wall, and a terminal portion having a boreaxially aligned with the nose bore, an electrode part disposed in thenose bore and having .a sparking surface at the firing end of saidinsulator, said electrode part including a cast body of a materialhaving a thermal conductivity of at least 0.5, at least a part of saidbody being with-in the nose bore and in closely adjacent spacedrelationship to the nose bore wall, and a sleeve disposed within anannular space bet-ween said body and the nose bore wall, out of contactwith and spaced from one by not more than approximately 0.001 inch andstructurally integral with the other, said sleeve comprising a metalselected from the group consisting of nickel, nickel alloys, iron,molybdenum and chromium, .whereby when the assembly is heated inservice, stressing of the nose bore wall by said body to an extentsufficient to cause insulator crack-ing is prevented.

9. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedin and supported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose extending from the firingend .to an intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore wall, and the terminal portion having a boreaxially aligned with the nose bore, an electrode part disposed in thenose bore and having a sparking surface at the firing end of saidinsulator, said electrode part including a cast silver body, at least apant of said body being in the nose bore and in closely adjacent spacedrela- .tionship to the nose bore wall, and means disposed within anannular space betwen said body and the nose bore wall, structurallyintegral with said body and out of contact with and spaced no more thanapproximately 0.001 inch from said nose bore wall, said means comprisinga material having a lower thermal conductivity than said cast silverbody and being effective to decrease the rate of heat transfer betwensaid body and the nose bore wall in an amount sufficient to preventstressing of the nose bore Wall by said body, when the assembly isheated in service, to an extent suflicient to cause insulator crackmg.

10. A spark plug assembly comprising a metal shell which is threaded forengagement in the combustion chamber of an engine, an insulator engagedand supported by said shell, said insulator having a terminal portionand a nose including a firing end, the nose eX tending from the firingend to an intermediate insulator area operatively associated with saidshell for conductive heat transfer between the two, said nose having abore defined by a nose bore wall, and a terminal portion having a boreaxially aligned with the nose bore, an electrode part disposed in thenose bore and having a sparking surface at the firing end of saidinsulator, said electrode part including a cast body of a materialhaving a thermal conductivity of at least 0.5, at least a part of saidbody being within the nose bore and in closely adjacent spacedrelationship .to the nose bore wall, and a sleeve disposed within anannular space between said body and the nose bore Wall, out of contactwith and spaced no more than approximately 0.001 inch from the nose borewall and structurally integral with said body, said sleeve compris- 10ing a metal selected from the group consisting of nickel, nickel alloys,iron, molybdenum and chromium, whereiby when the assembly is heated inservice, stressing of the nose bore wall by said body to an extentsufiicient to cause insulator cracking is prevented.

References Cited by the Examiner UNITED STATES PATENTS 2,081,500 5/ 1937 Nowosielski 313- X 2,226,415 12/ 1940 Kapp 313142 2,296,033 9/ 1942Heller 31311.5 2,360,287 10/ 1944 Smith 2925 1 2 2,717,438 9/ 1955Schwartzwolder et al. 29-25 .12 2,871,388 1/1959 Adair 313-141 FOREIGNPATENTS 885,466 5/ 1943 France. 886,455 7/ 1943 France. 558,977 1/ 1944Great Britain.

GEORGE N. WEST BY, Primary Examiner.

JOHN W. HUCKERT, Examiner.

V. LAFRA'NCHI, Assistant Examiner.

1. A SPARK PLUG ASSEMBLY COMPRISING A METAL SHELL WHICH IS THREADED FORENGAGEMENT IN THE COMBUSTION CHAMBER OF AN ENGINE, AN INSULATOR ENGAGEDAND SUPPORTED BY SAID SHELL, SAID INSULATOR HAVING A TERMINAL PORTIONAND A NOSE INCLUDING A FIRING END, THE NOSE EXTENDING FROM THE FIRING TOAN INTERMEDIATE INSULATOR AREA OPERATIVELY ASSOCIATED WITH SAID SHELLFOR CONDUCTIVE HEAT TRANSFER BETWEEN THE TWO, SAID NOSE HAVING A BOREDEFINED BY A NOSE BORE WALL, AND THE TERMINAL PORTION HAVING A BOREAXIALLY ALIGNED WITH THE NOSE BORE, AND ELECTRODE PART DISPOSED IN THENOSE BORE AND HAVING A SPARKING SURFACE IN THE VICINITY OF THE FIRINGEND OF SAID INSULATOR, SAID ELECTRODE PART INCLUDING A CAST BODY OF AMATERIAL HAVING A THERMAL CONDUCTIVITY OF AT LEAST 0.5, AT LEAST A PARTOF SAID BODY BEING WITHIN THE NOSE BORE AND IN CLOSELY ADJACENT, SPACEDRELATIONSHIP TO THE NOSE BORE WALL, AND